This magnetic survey contour map reveals the location of a buried, 16-inch-diameter gas pipeline. Sewers, utility lines, and fencing are among the features shown.
Geotechnical and hydrogeologic site evaluation often requires surface and near-surface soils characterization. This ground conductivity survey map over the footprint of a proposed manufacturing facility (blue box) illustrates the distribution of electrically conductive clay and clay-rich soils (blue, green, and yellow colors) with respect to electrically resistive sandy soils (red and pink colors).
To map measurements accurately, SwRI scientists survey ground conductivity with an electromagnetic system coupled to a global positioning system.
Delineating the subsurface distribution of geologic material and features (including faults and voids) and freshwater resources, and identifying contaminated zones and buried tanks play an important role in geotechnical site characterization, geologic and hydrologic resource exploration, and environmental site assessment and restoration. When conducting site characterization activities, exploratory borehole data are increasingly being supplemented with data from noninvasive, surface-based geophysical investigations to establish the need for and location of further site characterization (e.g., borings or test pits), thereby, reducing overall cost. Geophysical survey techniques provide a remote means of mapping subsurface geology, reservoirs, and contamination.
Geophysical Survey Techniques
The Geosciences and Engineering Division at Southwest Research Institute (SwRI) conducts integrated geophysical surveys to characterize the Earth's subsurface. Scientists and engineers with expertise and applied experience in geology, geophysics, and geolocation have developed cost-effective methods for comprehensive subsurface mapping using the following geophysical techniques:
- Electrical resistivity sounding and profiling (1-D through 3-D)
- Frequency-domain electromagnetics (e.g., ground conductivity)
- Time-domain electromagnetics
- Ground-penetrating radar
Geophysical methods are selected based upon site-specific criteria, including:
- Electromagnetic properties of and contrasts between subsurface materials
- Desired depth of investigation
- Desired resolution
Using state-of-the-science geophysical and geolocation instrumentation, software, and data processing methods (including joint inversions), Staff can rapidly and accurately locate, map, and interpret subsurface features and anomalies.
Site Characterization Applications
Geophysical survey techniques provide data at a variety of scales, from the regional geologic setting to the site-specific level, and these data can be applied to a range of environmental, natural resource, and geotechnical engineering site characterization needs.
Scientists have applied geophysical measurements and survey techniques to:
- Detection and mapping of geologic structure (lithology/fault detection and delineation)
- Cave and karst feature detection
- Reconnaissance and site-specific resource assessment for the aggregate industry (hard rock and unconsolidated sediments)
- Evaluation of soil and consolidated sediment for geotechnical design of foundations and seismic site response
- Aquifer characterization
- Delineation of depth to groundwater
- Water quality (salinity)
- Delineation of buried igneous features (e.g., dikes and volcanoes)
- Delineation of buried metal objects, pipelines, and electric utilities
- Leaking dam evaluation
Shallow karst features in limestone can affect building and foundation designs. This electrical resistivity profile illustrates the probable depth and extent of electrically resistive karst features, such as caves and cavities, beneath a retail development site.
Paleo-stream channel deposits are sources of sand and gravel for aggregate and may be shallow aquifers that can be developed for water supply. This electrical resistivity profile illustrates the depth, extent, and thickness of an electrically resistive paleo-stream channel deposited in a fluvial sedimentary environment.
geologic resource exploration • geotechnical site characterization • noninvasive geophysical measurements • surface-based measurements • geophysical survey techniques • subsurface geology maps • subsurface feature detection